Water, water everywhere, but not enough to drink

Published: Winter 2011-12

As climate change accelerates, worldwide fresh water supplies are predicted to become increasingly stressed. However, since oceans cover 70 percent of the planet, you might be skeptical of a water problem. With all that sea sloshing around, there should be enough for everybody, right? Just remove the salt. Problem solved.

Well, not quite. As much as desalination, making fresh water from saline sources such as the ocean, may seem the perfect answer to the world’s increasing fresh water needs, it is a limited answer, William Phillip of Notre Dame and Menachem Elimelech of Yale write in a recent Science article.

Great strides in desalination truly have been made in recent years, the chemical engineering professors say. Innovations such as improved membranes and better pumps have dramatically improved process efficiency, and large-scale reverse osmosis desalination plants are being constructed at a rapid pace. In fact, they note worldwide desalination capacity is projected to double by the year 2016.

Despite those gains, however, the researchers say the number of people living in water-stressed regions is projected to increase from one-third of the world’s population to two-thirds by the year 2025.

Phillip and Elimelech argue that desalination may be one tool to help solve the world’s fresh water needs, but it is does have its downside. For instance, desalination is probably the most energy-intensive method for increasing water supplies, and there isn’t much hope for improvement.

The Notre Dame and Yale professors point out that “reverse osmosis,” the most advanced desalination technology, has been tweaked so much in the past 40 years that it is near the theoretical and practical limits of its efficiency. Since the desalination process itself is about as good as it gets, they argue any future improvements are likely to come in the pre-treatment phase, where other contaminants, such as organic matter from decomposing seaweed, are filtered out.

“The main point we wanted to make is that desalination should be thought of as an option only after all other more sustainable fresh water sources, such as conservation and water recycling, have been exhausted. Just about any fresh water treatment technology is much less energy intensive than desalination,” Phillip says.